Since iTunes 9.2 is required to install iOS 4 it looks like the warning is a temporary one and users who want to jailbreak their iPhone will probably have to wait for the iPhone Dev Team to release their jailbreaking tools for iOS 4.

If you don’t care about jailbreaking then you can download iTunes 9.1.1 from this link or via Apple Software update. Please don’t forget to let us know how it goes.

Update:

Please note that if you've already jailbroken your iPhone, iPad or iPod Touch using Spirit then you can upgrade to iTunes 9.2. This warning is only applicable for those user who haven't jailbroken their iDevice yet and want to jailbreak using Spirit.